1. Field of the Invention
This invention relates to a flexible printed circuit board unit wherein electronic parts such as semiconductor devices are mounted on a flexible printed circuit board.
2. Description of the Related Art
A conventional circuit board unit of the type mentioned is disclosed in Japanese Patent Laid-Open No. 220736/1991 which has a structure shown in FIG. 1.
This circuit board unit 101 includes reinforcing plates 109 provided on the reverse side of flexible printed circuit board 102 on which semiconductor devices 106 are mounted. Reinforcing plates 109 are provided particularly on the reverse side of flexible printed circuit board 102 at locations at which semiconductor devices 106 are mounted in order to increase the rigidity of the portions of flexible printed circuit board 102 at the locations at which semiconductor devices 106 are mounted, thereby enhancing the reliability of electric connection between semiconductor devices 106 and flexible printed circuit board 102. Semiconductor devices 106 are encapsulated in encapsulating resin 108.
Japanese Patent Laid-Open No. 220736/1991 mentioned above further discloses circuit board unit 111 wherein, on a front surface of flexible printed circuit board 112 on which semiconductor devices 116 are mounted, reinforcing frames 119 which surround regions in which semiconductor devices 116 are mounted are provided as shown in FIG. 2. In the example shown, encapsulating resin 118 is flowed into and hardened in regions surrounded by reinforcing frames 119 to provide a rigidity to the portions of flexible printed circuit board 112 at which semiconductor devices 116 are mounted and simultaneously encapsulate semiconductor devices 116.
However, the conventional circuit board units described above do not sometimes present a sufficient reinforcing function to enhance the reliability of electric connection between the semiconductor devices and the flexible printed circuit board.
When an assembling process for a circuit board unit is taken into consideration, an organic material such as a glass epoxy material is preferably used for reinforcing members for a flexible printed circuit board. However, organic materials do not provide a sufficient rigidity. If an organic material of a comparatively great thickness is used to assure a sufficient rigidity, then a great difference in thickness will be produced between a location of the flexible printed circuit board at which the reinforcing member is provided and another location at which the reinforcing member is not provided. This makes handling of the flexible printed circuit board in the assembling process difficult. Thus, it may be proposed to alternatively use, for the reinforcing member, a metal plate having a sufficient rigidity with a comparatively small thickness. In this instance, however, since the metal plate is heavier than an organic material, it is difficult to transport the circuit board unit in the assembling process. Further, since the metal plate has a high thermal conductivity, condition setting such as temperature setting when a semiconductor device is to be connected to the flexible printed circuit board becomes severe.
In short, if it is tried to assure a high rigidity of a flexible printed circuit board in order to stabilize the electric connection between semiconductor devices and the flexible printed circuit board, then such disadvantages as described above occur in the assembling process.